They ask you to hasten in bringing chastisement upon them. Had there not been an appointed term for it, the chastisement would have already visited them; in fact it will come down upon them all of a sudden (at its appointed time) while they will not be aware of it.
वे तुमसे यातना के लिए जल्दी मचा रहे है। यदि इसका एक नियत समय न होता तो उनपर अवश्य ही यातना आ जाती। वह तो अचानक उनपर आकर रहेगी कि उन्हें ख़बर भी न होगी